In photographic processing solutions, the concentrations of the various components need to be kept close to certain aim levels. The normal way to maintain chemical levels is to replenish the components at the same rate as they are consumed. Consumption rates vary with exposure (and hence the amount of silver developed), and losses due to aerial oxidation, evaporation, etc., can vary widely for different processes and processing machines. Even with a particular machine and process, simply replenishing at a calculated rate can lead to undesirable variability. It is normal for processing chemicals to be as soluble as possible to facilitate solution preparation.
The problem to be solved is how to maintain the desired concentration of a processing solution component in a simple but accurate way.